Medical procedures involving insertion of a medical device into the brain (through a burr hole formed in the skull) are used to treat a variety of medical conditions. For example, electrical stimulation of the brain to relieve chronic pain, or for the treatment of movement disorders, may necessitate the implantation, via the burr hole, of an electrode or lead. Similarly, burr holes are typically formed to allow implantation of a therapy catheter, e.g., an intraparenchymal (IPA) or intracerebroventricular catheter, to treat various ailments.
Use of such devices to deliver therapy to the brain generally involves the insertion of the device into the brain through the burr hole and positioning a distal, therapy delivery tip of the device at a desired target tissue location. During a typical implantation procedure, an incision is made in the scalp to expose the patient's skull. After forming a burr hole through the skull, the device is inserted into the brain. To accurately place the device, surgeons typically use stereotactic apparatus/procedures. One exemplary stereotactic apparatus is described in U.S. Pat. No. 4,350,159 to Gouda, which may be used to position, for example, an electrode.
As one can appreciate, once an inserted device such as a catheter is properly positioned, it is important that it be adequately immobilized to prevent movement of its distal tip from its intended location. Such movement may result in undesirable lateral forces applied by the implanted medical device to brain tissue, especially near the entry point (cortex). Moreover, even minimal movement of the device's distal tip may reduce therapeutic efficacy. Accordingly, reliable methods and apparatus for anchoring and securing the device relative to the burr hole are desirable.
After locating the distal tip at the target tissue location, a portion of the medical device that extends outside of the burr hole may be anchored using an anchor device. A proximal end of the medical device may then connect to a therapeutic source (e.g., for a catheter, to a reservoir containing a therapeutic agent; for a lead, to an electrical stimulation source). For example, when the medical device is a therapy catheter, the proximal end of the therapy catheter may connect to a second, delivery or pump catheter that is, in turn, coupled to an implantable pump containing the therapeutic agent. As a result, the agent may be delivered through the delivery catheter and the therapy catheter to the desired target tissue location within the patient.
Increasingly, surgeons desire access to the brain via device trajectories that are angled relative to the burr hole. That is, some surgeries may benefit from insertion of the medical device into the brain at an angle that is canted from (e.g., not aligned with) an axis normal to the skull surface at the burr hole. Many existing burr hole anchors, however, are configured to grip or secure the medical device assuming that its orientation is normal to the skull surface. In the case of an angled implant trajectory, such anchors may impart clamping forces that potentially bias the device away from its original implant trajectory, and thus bias the therapy delivery tip away from the intended target tissue location. This result may be amplified with increased trajectory angle, stiffer medical devices, and shallower insertion depths.